Electronically tunable acousto-optic bandpass filters have been constructed so that a cone of light of first polarization is diffracted by an acoustic wave in an anisotropic medium (such as a birefringent crystal) to shift from the first to a second polarization of the light beam at a selected bandpass of optical frequencies. The center wavelength of the passband of the acousto-optic filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
Two basic types of tunable acousto-optic filters have been constructed: collinear and noncollinear. In the collinear filter, the incident and diffracted light beams inside the birefringent crystal are collinear with the acoustic beam. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. The collinear type of acousto-optic filter is disclosed in an article entitled "Acousto-Optic Tunable Filters" appearing on pages 744-747 in the June, 1969 issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288, entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear filter, the light beams inside the birefringent crystal are noncollinear with the acoustic beam. The diffracted light beam at the passband is selected from the incident light beam by either crossed polarizers or spatial separations. The noncolhinear type of acousto-optic filter is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture", appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121, entitled "Non-collinear Tunable Acousto-Optic Filter".
The most significant feature of both the collinear and noncollinear tunable acousto-optic filter is that a narrow filter bandpass can be maintained for a relatively large cone of incident light. This large angular aperture characteristic is due to the proper choice of acousto-optic interaction geometry, wherein the tangents to the locus of the incident and diffracted light wave vectors are parallel. This condition of "parallel tangents" applies to both noncollinear and collinear acousto-optic filters.
For the remainder of this disclosure, "tunable acousto-optic filter" will be defined as an optical filter that operates on the basis of the "parallel tangents" type of acousto-optic diffraction in a anisotropic medium wherein the "parallel tangents" condition is satisfied.
In one kind of collinear acousto-optic filter, the tuning of the passband is accomplished by varying the birefringence of the crystal with the application of a voltage applied to two electrodes placed along the length of the birefringent crystal. This kind of voltage-tuned collinear acousto-optic filter is disclosed in an article entitled "Influence of Electrostatic Fields on the Properties of Acoustically Tuned Optical Filters" appearing on pages 439-441 of the May 1, 1977 issue of Applied Physics Letters (Vol. 30) and in U.S. Pat. No. 3,701,584, entitled "Tuned Voltage Variable Birefringent Acousto-Optic Filter".
The prior art described above is concerned mainly with an acousto-optic apparatus that is useful as an electronically tunable optical filter wherein the tuning of the filter is achieved by changing acoustic frequency or electrically induced birefringence. Such an anisotropic acousto-optic apparatus could also be used in an optical modulator. Since the light transmission through the acousto-optic filter is a function of the power of the acoustic wave, a most direct way to obtain intensity modulation of light is to amplitude-modulate the acoustic wave. This kind of collinear acousto-optic filter used as a light modulator is disclosed in U.S. Pat. No. 3,632,193, entitled "Method and Apparatus for Control of Light Transmission Through an Anisotropic Medium".
It is important to note that the collinear acousto-optic filter used as a light modulator operates equally on light generated from a coherent source (such as the laser) or from an incoherent source (such as a tungsten lamp). However, for some applications it is desirable to employ a laser beam modulator that is insensitive to incoherent light. One such application is the detection of a weak laser beam in the presence of incoherent background radiation such as sunlight.
The primary object of this invention, therefore, is to provide a light modulation method and apparatus that is sensitive to the temporal coherence of the light beam.